Fix flaky failure_distributed_results

Sometimes in CI we run into this failure:

```diff
   SELECT resultId, nodeport, rowcount, targetShardId, targetShardIndex
   FROM partition_task_list_results('test', $$ SELECT * FROM source_table $$, 'target_table')
           NATURAL JOIN pg_dist_node;
-WARNING:  connection to the remote node localhost:xxxxx failed with the following error: connection not open
+ERROR:  connection to the remote node localhost:9060 failed with the following error: connection not open
 SELECT * FROM distributed_result_info ORDER BY resultId;
-       resultid        | nodeport | rowcount | targetshardid | targetshardindex
----------------------------------------------------------------------
- test_from_100800_to_0 |     9060 |       22 |        100805 |                0
- test_from_100801_to_0 |    57637 |        2 |        100805 |                0
- test_from_100801_to_1 |    57637 |       15 |        100806 |                1
- test_from_100802_to_1 |    57637 |       10 |        100806 |                1
- test_from_100802_to_2 |    57637 |        5 |        100807 |                2
- test_from_100803_to_2 |    57637 |       18 |        100807 |                2
- test_from_100803_to_3 |    57637 |        4 |        100808 |                3
- test_from_100804_to_3 |     9060 |       24 |        100808 |                3
-(8 rows)
-
+ERROR:  current transaction is aborted, commands ignored until end of transaction block
 -- fetch from worker 2 should fail
 SAVEPOINT s1;
+ERROR:  current transaction is aborted, commands ignored until end of transaction block
 SELECT fetch_intermediate_results('{test_from_100802_to_1,test_from_100802_to_2}'::text[], 'localhost', :worker_2_port) > 0 AS fetched;
-ERROR:  could not open file "base/pgsql_job_cache/xx_x_xxx/test_from_100802_to_1.data": No such file or directory
-CONTEXT:  while executing command on localhost:xxxxx
+ERROR:  current transaction is aborted, commands ignored until end of transaction block
 ROLLBACK TO SAVEPOINT s1;
+ERROR:  savepoint "s1" does not exist
 -- fetch from worker 1 should succeed
 SELECT fetch_intermediate_results('{test_from_100802_to_1,test_from_100802_to_2}'::text[], 'localhost', :worker_1_port) > 0 AS fetched;
- fetched
----------------------------------------------------------------------
- t
-(1 row)
-
+ERROR:  current transaction is aborted, commands ignored until end of transaction block
 -- make sure the results read are same as the previous transaction block
 SELECT count(*), sum(x) FROM
   read_intermediate_results('{test_from_100802_to_1,test_from_100802_to_2}'::text[],'binary') AS res (x int);
- count | sum
----------------------------------------------------------------------
-    15 | 863
-(1 row)
-
+ERROR:  current transaction is aborted, commands ignored until end of transaction block
 ROLLBACk;
```

As outlined in the #7306 I created, the reason for this is related to
only having a single connection open to the node. Finding and fixing the
full cause is not trivial, so instead this PR starts working around it
by forcing maximium paralism.

src/test/regress/expected/failure_distributed_results.out

@@ -14,6 +14,8 @@ SELECT citus.mitmproxy('conn.allow()');
 (1 row)
 
 SET citus.next_shard_id TO 100800;
+-- Needed because of issue #7306
+SET citus.force_max_query_parallelization TO true;
 -- always try the 1st replica before the 2nd replica.
 SET citus.task_assignment_policy TO 'first-replica';
 --

src/test/regress/sql/failure_distributed_results.sql

@@ -15,6 +15,8 @@ SET client_min_messages TO WARNING;
 SELECT citus.mitmproxy('conn.allow()');
 
 SET citus.next_shard_id TO 100800;
+-- Needed because of issue #7306
+SET citus.force_max_query_parallelization TO true;
 
 -- always try the 1st replica before the 2nd replica.
 SET citus.task_assignment_policy TO 'first-replica';